Chrom - Robin was not in the E3 demo and he's also the only Fire Emblem character whose Final Smash wasn't shown at all by now, and with the whole Echo Fighters thing existing he's obviously coming as an Ike Echo.

Dark Samus - Ridley's victory theme is a darker version of the Metroid victory theme (like what Bowser and Bowser Jr. have) as opposed to his own theme (like Dark Pit), and Samus' Dark Samus palette is gone, which also happened with Dark Pit. Add in the whole echo fighters thing and her AT not being seen at E3, and she's commonly seen as the most likely newcomer.

Shadow - His AT hasn't been seen either, and now Knuckles is one, which seems to indicate that he's also an echo fighter since some people seem to think third-parties can't have more than one AT.

Simon Belmont - Because of Vergeben despite coming from a batch of inaccurate information. Also, they wouldn't bring Snake back and make Bomberman an AT without giving Konami another playable character, would they?

Banjo and Kazooie - Because of Minecraft cross-play. Also, Phil Spencer tweeted that he'd be fine with it, so it's obviously a done deal.

Sakurai hasn't once mentioned DLC, and for all we know, he may call it a day once development ends, but that hasn't stopped people from assuming which characters will show up even before we know the full base roster.